Laser engraving system

ABSTRACT

A laser engraving machine (4) is used for engraving a workpiece surface (22) by a modulated laser beam in order to form a desired profile in the workpiece surface. The fine structures of the profile are formed by the laser beam of a first laser which is modulated by an acoustooptic modulator (12) with relatively high modulation frequency, while the deep areas of the desired profile are formed by the laser beam of a second laser (10), for which purpose the modulator (12), on the one hand, and the second laser beam source (10), on the other hand, are driven by interrelated but separate control signals (S3, S2). The two perpendicular polarized laser beams from the modulator (12) and the second laser beam source (10) are transmitted and reflected by a selective mirror (14), respectively, and applied commonly via an optical system (18) to the workpiece surface (22) to be machined.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a laser engraving machine for engraving aworkpiece surface, having a first laser beam source, a modulator in thebeam path of the first laser beam source, an optical system followingthe modulator and spaced from the workpiece surface, the optical systemand the workpiece being moved relative to each other, a control devicedriving the modulator with a first control signal so that its outputradiation impinging on the workpiece surface is modulated according tothe first control signal and machines the workpiece surface accordinglydeeply, and a second laser beam source which is driven by the controldevice with a second control signal.

2. Description of the Related Art

Such laser engraving machines are known. The laser beam sourcecharacteristically consists of a CO₂ laser which is driven by thecontrol device with a control signal which depends on the desiredmachining profile of the workpiece surface. The workpiece surface ismoved relative to the optical system. The workpiece ischaracteristically a cylinder made of rubber or plastic for example,which is clamped in a laser engraving machine where it is rotated andsimultaneously moved translationally parallel to its rotation axis sothat the laser beam focused on the workpiece surface by the opticalsystem scans the workpiece surface and removes the workpiece surfacemore or less deeply in accordance with the control signal bycorresponding beam intensity.

FIG. 1 shows a laser engraving machine which partly corresponds to theprior art.

Workpiece 2 is a cylinder made of rubber or having rubber surface 22.Workpiece 2 is rotated and moved translationally in the direction of thearrows.

Stationary laser engraving machine 4 contains a PC (personal computer)with interface IF as a control device and signal generator. Controldevice 6 delivers to first CO₂ laser 8 control signal 1 which causesfirst laser 8 to apply output laser radiation to acoustooptic modulator12, this output laser radiation of first laser 8 being linearlypolarized and having an unvarying amplitude.

Via interface IF of control device 6 control signal S3 is applied toacoustooptic modulator 12, causing modulator 12 to modulate the laserbeam from first laser 8 in accordance with the signal fluctuations ofcontrol signal S3 such that the output laser beam from acoustoopticmodulator 12 has fluctuations of intensity corresponding to controlsignal S3.

The structure and use of acoustooptic modulator 12 are known inprinciple. It characteristically involves a crystal and a piezoelectricelement so that when the piezoelectric element is driven acoustic wavesare sent through the crystal which influence its optical properties. Thelaser beam passing through the modulator is diffracted in accordancewith the frequency of the acoustic wave, i.e. the power or intensity ofthe output beam from modulator 12 is modulated, whereby working beam 1emerging from the modulator is the beam of the first order ofdiffraction of the modulator.

The output laser beam from acoustooptic modulator 12 passes via opticalsystem 18, where the radiation is focused, onto workpiece surface 22.

The power of the laser beam impinging on workpiece surface 22 thusvaries in accordance with control signal S3, and since this laser beamscans workpiece surface 22 at constant speed, a profile corresponding tocontrol signal S3 arises in workpiece surface 22. High beam powerresults in great machining depth, low beam power in small machiningdepth.

A laser engraving machine of the abovementioned kind, as is known fromDE 42 12 390 A1, is constructed such that the two laser beams from thelaser beam sources are directed before the workpiece via separateoptical paths onto the workpiece surface so as to produce there a beamspot condensed by exact overlap of several beams, or else a multipartbeam spot in which several partial beams are combined in a certainpattern in partial overlap or no overlap at all. U.S. Pat. No. 4,947,023discloses a laser engraving machine wherein two laser beams are guidedcoaxially before the optical system in a partial area of theirparticular optical paths. This is intended to obtain a double engravingdepth in the workpiece if both lasers are switched on during the workcycle.

DE 37 14 504 A1 discloses a laser engraving machine using two laserswith different wavelengths. The two laser beams of different wavelengthare brought together on one machining spot.

It is known to modulate directly the operation of first laser 8 with theaid of control device 6 to obtain the desired machining profile onworkpiece surface 22. However, a typical CO₂ laser has a maximummodulation frequency in the kilohertz range, which prohibits fastmachining of workpiece surfaces at least when the desired profile hasvery fine structures.

The use of the acoustooptic modulator permits faster and finer machiningof the workpiece surface, because typical acoustooptic modulators have amaximum modulation frequency in the megahertz range.

However, the use of acoustooptic modulators is restricted because such amodulator can usually only modulate a laser beam with a maximum power of100 watts. In many cases of application, for example when graduatingpress cylinders, it is imperative to obtain a certain profile depth. Ata given upper power limit due to the modulator and at a given minimumprofile depth at the bottom of the profile formed in workpiece surface22 the machining speed is consequently restricted, since a certainminimum energy must be applied to the workpiece surface via the laserbeam to attain the required profile depth in the workpiece surface. Itfollows that the laser beam having maximum power can only be movedacross the workpiece surface relatively slowly.

The invention is based on the problem of providing a laser engravingmachine of the abovementioned kind which can form fine contours, on theone hand, and obtain a certain minimum profile depth, on the other hand,while machining the workpiece surface fast.

SUMMARY OF THE INVENTION

This problem is solved according to the invention in a laser engravingmachine of the stated kind in that the output laser radiation from thesecond laser beam source is brought together with the output laserradiation from the modulator on a common beam axis before the opticalsystem, and the first and second control signals are shaped such thatthe first control signal defines the fine structures of the desiredprofile while the second control signal supplied to the second laserbeam source corresponds to the deep places in the profile.

The control signals used for driving the laser beam sources do not havea rectangular form but sloping signal edges, i.e. they change theirlevel between the maximum and minimal values gradually. Only to form theedges of fine structures is there a fast change in the signal level ofthe first control signal for the modulator, so that the output laserradiation from the modulator likewise changes abruptly. Thus modulatedlaser beams form fine contours in the workpiece surface. When the laserbeam is then guided further across the workpiece surface, the power ofthe laser beam increases so that the profile produced in the workpiecesurface becomes accordingly deeper. While the first part of an engravedarea is formed solely by the laser beam emitted by the modulator, thelaser beam from the second laser beam source is then added. Thisadditional laser beam makes the total power of the laser beam impingingon the workpiece surface greater, preferably many times greater, thanthe power which delivered only by the output laser beam from themodulator.

While the fine contours are produced by the relatively high modulationfrequency of the acoustooptic modulator, the deep areas of the desiredprofile are formed by connecting the second laser beam source. Since thedeep areas must only be produced in those portions of the profile whichare relatively long (viewed along the scanning line on the workpiecesurface), the maximum modulation frequency in the kilohertz rangesuffices for the second laser light source.

The inventive measure thus utilizes the high maximum modulationfrequency of an acoustooptic modulator, on the one hand, and compensatesits restriction by the maximum laser power by connecting the secondlaser beam source when deep areas of the profile must be produced, onthe other hand.

The invention can fundamentally be realized with two separate lasers,for example CO₂ lasers. In a practical embodiment, a laser with twolaser tubes is used, one laser tube delivering a laser beam withconstant amplitude while the laser beam of the other laser tube isguided through the acoustooptic modulator.

One obtains an embodiment which is especially favorable in practice whenthe laser beams delivered by the first and second laser beam sources arelinearly polarized perpendicular to each other. The first linearlypolarized laser beam is applied to the acoustooptic modulator andmodulated thereby with high modulation frequency. The output laser beamfrom the modulator is applied to a selective mirror disposed in the beampath at an angle of 45°. The polarization vector of this laser beam fromthe modulator has an angle of about 45° to the mirror surface on whichthe laser beam impinges. The laser beam is transmitted by the mirror.The other laser beam having a polarization vector perpendicular to theformer polarization vector hits the mirror from the other side, itspolarization vector extending parallel to the mirror surface inquestion. This causes the laser beam from the second laser beam sourceto be reflected by the mirror. The transmitted and reflected, eachlinearly polarized, laser beams are combined into a common laser beamwhich passes onto the optical system and is focused thereby onto theworkpiece surface.

In the way known in the art, the control device of such a laserengraving machine consists mainly of a PC (personal computer) andcorresponding interface IF. Data of the desired profile for theworkpiece surface to be engraved are stored in a memory. These data areprocessed by the PC into a control signal, that is, a signal varying intime between two levels whereby the signal level correspondsfundamentally to the desired profile along the scanning line on theworkpiece surface. This control signal is divided according to theinvention into the first control signal supplied to the acoustoopticmodulator, and the second control signal supplied to the second laserbeam source. Since the laser power emitted by the modulator, like thelaser power emitted by the second laser beam source, is essentiallyproportional to the amplitude characteristic of the control signal inquestion, the sum of the first and second control signals yields thecontrol signal initially provided by the PC. In other words, the firstand second control signals result from suitable subtractive separationof the control signal produced by the PC.

The modulator can preferably be an acoustooptic modulator following thelaser. However, a similar mode of operation is also achieved if anothermodulator is used. One can also obtain the desired modulation of thelaser light for machining the workpiece surface with the aid of aQ-switch in the laser resonator.

The first laser beam source and second laser beam source need notnecessarily produce laser light with the same wavelength. One can alsouse different wavelengths for the first and second lasers. In this case,a wavelength-selective mirror is then disposed before the optical systemof the laser engraving machine.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following some embodiments of the invention will be explainedmore closely with reference to the drawing, in which:

FIG. 1 shows a schematic sketch of a laser engraving machine;

FIG. 2 shows pulse diagrams of several control signal patterns inconjunction with a corresponding profile of a workpiece surface,

FIG. 3 shows a schematic representation of a further embodiment of alaser engraving machine according to the invention, and

FIG. 4 shows a schematic representation of an embodiment modified overFIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Inventive laser engraving machine 4 shown in FIG. 1 contains theabove-explained components of a known laser engraving machine, namelycontrol device 6 consisting of a PC and interface IF, first laser 8,acoustooptic modulator 12 and optical system 18 which focuses the outputlaser beam from acoustooptic modulator (AOM) 12 onto workpiece surface22 of workpiece 2, which is a rubber cylinder to be engraved.

FIG. 1 shows by a dash-dot line an additional part to be referred tohere as depth engraving additional part 16. The depth engravingadditional part contains selective mirror 14 and second laser 10.

Control device 6 delivers control signal S1 to first laser 8 so that thelatter applies a laser beam with linear polarization and constant powerto accustooptic modulator 12. The latter receives from control device 6control signal S3 defining the fine contours of the desired profile ofworkpiece surface 22. The likewise linearly polarized output laser beamfrom acoustooptic modulator 12 is transmitted by selective mirror 14 andpasses via optical system 18 onto workpiece surface 22.

Second laser 10 receives control signal S2 from PC 6 and delivers alaser output beam which is linearly polarized but the polarizationvector of the laser beam emitted by second laser 10 is perpendicular tothe polarization vector of the laser beam delivered by modulator 12. Dueto the polarization vectors indicated by short lines and dots in FIG. 1,selective mirror 14 transmits the laser beam emitted by modulator 12 andreflects the laser beam delivered by second laser 10, so that the twolaser beams having perpendicular polarization vectors are united in acommon beam path and reach workpiece surface 22 via optical system 18.

While control signal S1 and therefore also the output laser beam fromfirst laser 8 have a constant amplitude or power, first control signalS3 and second control signal S2 have a time behavior corresponding tothe desired profile along the scanning line of the laser beam onworkpiece surface 22. The power of the laser beams emitted by modulator12 and second laser 10 is virtually proportional to the amplitude ofcontrol signals S3 and S2, respectively.

FIG. 2 shows at A) the profile of workpiece 2 in its workpiece surface22. The course of the profile along the scanning line of the laser beamon the workpiece surface is of course related to the scanning speed atwhich the laser beam moves across the workpiece surface. The localchange of the profile in workpiece surface 22 thus corresponds to thetime rate of change of (first) control signal S3 supplied toacoustooptic modulator 12, and of (second) control signal S2 supplied tosecond laser 10 by control device 6. Control signals S1, S3 and S2 areshown in FIG. 2 at B), C) and D).

Due to the abovementioned relation between the course of the profilealong the scanning line of the laser beam on workpiece surface 22 andthe time behavior of the control signals, in particular control signalsS3 and S2, individual places in the profile shown in FIG. 2A) can berelated to times t₁ to t₆ during the course of the control signals.

Control signal S1 supplied to first laser 8 has constant (maximum)amplitude so that laser 8 delivers a laser beam with constant power toacoustooptic modulator 12.

In the following the course of first control signal S3 (FIG. 2C)supplied to acoustooptic modulator 12 will be considered. At time t₁ thelevel of control signal S3 jumps to a certain value, and the resultingabrupt increase in power of the laser beam emitted by modulator 12causes a steep step to form in workpiece surface 22. Between times t₁and t₂ the level of control signal S3 increases gradually to a maximumvalue, and the depth of the profile accordingly increases gradually withrespect to workpiece surface 22. At time t₂ when the maximum level ofcontrol signal S3 and therefore the maximum power of the laser beamdelivered by acoustooptic modulator 12 is reached, the level of controlsignal S2 begins to rise. In accordance with the sum of the levels ofcontrol signals S2 and S3 the power of the laser beams united byselective mirror 14 increases on the workpiece surface so that theprofile depth is increased. At time t₃ the level of control signal S2reaches its maximum value. This corresponds to maximum profile depthT_(G). Total profile depth T_(G) results from the sum of the maximumamplitudes of the two control signals S2 and S3.

At time t₄ the level of control signal S2 begins to sink, and the depthof the profile in the workpiece surface accordingly decreases. Betweent₂ and t₅ control signal S3 remains at its maximum level, beginning todecrease after time t₅ when control signal S2 has resumed zero level,until the level of control signal S3 drops to zero at time t₆. Aftertime t₆ no laser beam passes onto the workpiece surface so that noengraving takes place.

Comparison of control signals S3 and S2 indicates that second controlsignal S2 supplied to second laser 10 is "turned on" only when theprofile is deeper than certain profile depth X in FIG. 2A). Consequentlythe "modulation frequency" required for second laser 10 is lower thanthe modulation frequency of acoustooptic modulator 12. The laser beamdelivered by latter modulator 12 thus ensures the formation of finecontours. This output laser beam from acoustooptic modulator 12 couldobtain a maximum machining depth of T_(M) (see FIG. 2A). At greaterdepths second laser 10 is connected.

FIG. 3 shows a further, special embodiment of a laser engraving machine.While the embodiment of FIG. 1 fundamentally uses two separate lasers,for example two separate CO₂ lasers, the embodiment of FIG. 3 uses laser30 with two laser tubes each emitting linearly polarized laserradiation, the two polarization vectors being perpendicular to eachother. At the output of laser 30 the laser light emerges with twoperpendicular components. By repeated deflection on tilted mirrors M1,M2 this light passes onto selective mirror M3 corresponding to mirror 14shown in FIG. 1. The radiation with the linear polarization, which isindicated by dots in FIG. 3, is reflected by mirror M3 onto opticalsystem 18. The radiation polarized perpendicular thereto reachesacoustooptic modulator 12 via further tilted mirror M4. The laserradiation emerging there passes via further tilted mirrors M5, M6through mirror M3 onto optical system 18. On mirror M5 there is afade-out of the zero order diffraction maxima. This radiation fractionis absorbed by absorber 20. The first order diffraction maximum reachesthe surface of workpiece 2 via the following mirrors and optical system18.

The above described embodiments can be modified within the scope ofprotection of the invention.

Instead of mirror M3 shown in FIG. 3 one can also use a Brewster windowwith a somewhat different geometry of the beam control, as shown in FIG.4 at B3.

Furthermore, one can also superimpose two laser beams with differentwavelengths by means of a wavelength-selective mirror. This embodimentis not shown in the drawing. However, it is clear that the two laserbeam sources can emit laser beams with different wavelengths in thisembodiment.

In the embodiment of FIG. 1 acoustooptic modulator 12 follows firstlaser 8. However, modulation of the laser light can also be obtained bya laser with a Q-switch. As known, this Q-switch is located in theresonator of the laser.

What is claimed is:
 1. A laser engraving machine (4) for engraving aworkpiece surface (22) of a workpiece, having a first laser beam source(8), a modulator (12) in a beam path of the first laser beam source (8),a following optical system (18) spaced from the workpiece surface (22),the optical system (18) and the workpiece being moved relative to eachother, a control device (6) which drives the modulator (12) with a firstcontrol signal (S3) so that its output laser radiation impinging on theworkpiece surface (22) is modulated according to the first controlsignal (S3) and machines the workpiece surface (22) to depths accordingto the first control signal (S3), and a second laser beam source (10)which is directly driven by the control device (6) with a second controlsignal (S2), characterized in that the output laser radiation from thesecond laser beam source (10) is brought together with the output laserradiation from the modulator (12) on a common beam axis before theoptical system (18), and the first and second control signals (S3, S2)are shaped such that the first control signal (33) defines finestructures of a desired profile while the second control signal (S2)supplied directly to the second laser beam source (10) corresponds todeep places in the profile.
 2. The laser engraving machine of claim 1,characterized in that the first and second laser beam sources (8, 10)each emit linearly polarized radiation with substantially perpendicularpolarization vectors, and the output laser radiation from the modulator(12) and the output laser radiation from the second laser beam source(10) are applied to a selective mirror (14) or a Brewster window (B3)from different sides so that the transmitted and reflected radiation isapplied commonly to the optical system.
 3. The laser engraving machineof claim 1, characterized in that the first and second laser beamsources emit laser light with different wavelengths, and these two laserbeams are applied to the optical system via a wavelength-selectivemirror.
 4. The laser engraving machine of any of claims 1, characterizedin that the first control signal (S3) and the second control signal (S2)are gained subtractively from a single control signal which correspondsto the desired profile of the machined workpiece surface (22).
 5. Thelaser engraving machine of any of claims 1, characterized in that themodulator is an acoustooptic modulator (12).
 6. The laser engravingmachine of any of claims 1, characterized in that the modulator isformed as a Q-switch of the first laser beam source (8).